You need to go into your server files (Files -> FTP File access on Multicraft) and open the file bukkit.yml.
The values that you need to edit are in this section:
The numbers indicate the total amount of mobs that can spawn in your world. Edit the values as you see fit, then save the file and restart the server.
The more monsters or animals you have on the server the more of a performance impact you will have. This means that your server will start lagging if you have too many monsters or animals.
Certain plugins override these mob spawning limits, if these limits don’t affect your server then it means that you have a plugin that sets the limits.